DE 42 39 361 A1 discloses a generic immersion-proof hydraulic unit comprising a connecting path that leads from the cavity in the direction of free ambience, and including a non-return valve that opens in the direction of an exhaust of the connecting path. In particular pumps with oscillating pump pistons produce a defined quantity of leakage fluid that can accumulate in a cavity designed as a crank chamber. The prior art unit allows the discharge of leakage fluid out of the cavity into the ambience in addition to a bleeding operation. The connecting path is isolated from the ambience in the reverse direction.